<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width:0;
            height:0;
            border-width:100px;
            border-style: solid;
            border-color:transparent transparent  red transparent;
            /* transform:rotate(90deg); */
            /* width: 0; 
            height: 0; 
            border-width: 100px; 
            border-style: solid; 
            border-color: transparent #0099CC transparent transparent; 
            transform: rotate(90deg);  */

        }
    </style>
</head>
<body>
    <div></div>
</body>
</html>